Introduction

The rapid shallow breathing index (RSBI) is the ratio determined by the frequency
(f) divided by the tidal volume (VT). An RSBI <105 has been widely accepted by healthcare
professionals as a criteria for weaning to extubation and has been integrated into
most mechanical ventilation weaning protocols. We hypothesized that the converse of
using the RSBI for weaning might be useful in predicting the need for noninvasive
ventilation. Advancements in technology have made it easier to accurately attain bedside
RSBI measurements. The purpose of this study was to ascertain a threshold value of
RSBI that could predict the need for noninvasive ventilation (NIV) in patients presenting
with acute respiratory distress to the critical care area (Cat 1) in the emergency
department.

Methods

This was a blinded, observational cohort trial that was approved by the Henry Ford
Hospital Institutional Review Board. Henry Ford Hospital is an urban, tertiary institution
in Detroit, Michigan with an emergency department census of 95,000 patient visits
per year. Inclusion criteria: patients > 18 years of age triaged to Cat 1 with acute
respiratory distress and for whom the decision to intubate, use NIV or discharge the
patient had not been decided. Exclusion criteria: immediate intubation, NIV, or discharge
from Cat 1. Baseline demographics and vital signs were collected prior to the initiation
of the trial (Figure 1). The CO2SMO Plus! with the ETCO2/flow sensor was used for obtaining bedside measurements. Patients would breathe through
the ETCO2/flow sensor for 60 seconds with nose clips.

Results

The threshold value for RSBI that discriminated best between no NIV and the need for
NIV was determined in 61 patients. Thirty-five patients who did not require ventilatory
support had a mean RSBI of 105, and 26 patients with NIV had a mean RSBI of 222 (P = 0.0001). A receiver-operating-characteristic curve was constructed based upon the
dataset in increments of 10 for the RSBI (Figure 2). An RSBI > 120 yielded a sensitivity of 0.81 and a specificity of 0.74 for determining
the need for NIV. A likelihood ratio positive (LR+) of 3.14 further illustrates the
formidable predictive value of the 120 RSBI.